Date: SisFrance dating of old earthquakes is based on the chronology of the Gregorian calendar (in France, 1582 reform) and takes into account ways of beginning the year (Christmas, Annunciation and Eater) used in various provinces until the 15th century.e.g.: The earthquake on 1st March 1489 (Julian calendar) is listed as 1st March 1490 (Gregorian calendar).

Similarly, SisFrance establishes the correspondence between Republican and Gregorian calendars.e.g.: The earthquake on 6 Pluviôse (fifth month in the Republican calendar) year 7 (Republican calendar) is dated 25 January 1799 (Gregorian calendar).

FALSE EARTHQUAKE: Event reported by accounts as a real earthquake but for which, after verification of the sources, the nature is similar to another type of phenomenon (landslide, collapse, mining rock burst or falling meteorite); in other cases, this may be the observance of an error in dating by an author of little faith.

DUBIOUS EARTHQUAKE: Event for which the arguments are insufficient to show its tectonic origin: date and location have not been able to be matched up and verified by various accounts; in other cases, the context (hurricane, violent storm) favour uncertainty.

It is assessed using a macro-seismic map where all the intensities determined in various places are reported, according to the observations available. The epicentral intensity provides the means for expressing the size of a past earthquake. The intensity is generally greatest at the epicentre and decreases as one moves away from the epicentre. The rate of decrease is greater when the focus of the earthquake is not very deep. Analysis of all the intensities observed on an earthquake provides the means for determining the magnitude which provides an estimate of the energy dissipated at the focus in the form of seismic waves. For current earthquakes, statistical correlations may be established between intensity and magnitude according to the depth of the focus. These correlations in return provide the means for estimating the likely magnitude of past earthquakes.

MACRO-SEISMIC INTENSITY: The macro-seismic intensity is the quantification of an earthquake's power at a particular point on the earth's surface, from a statistical estimate of the effects caused at that place, on people, buildings and the environment.In SisFrance, the absence of an intensity value for a location corresponds to the fact that the details on the effects of the earthquake are lacking for that place. The earthquake is then considered as having been felt: in a certain manner (code A), fairly certain (code B)or uncertain (code C).

The intensity is assessed on a macro-seismic scale.In France and in most European countries, the intensity is expressed on the M.S.K. 1964 scale (from the name of the people who devised it: Medvedev, Sponheuer and Karnik), which has 12 degrees expressed in Roman numerals to highlight the discrete nature of the degrees on the scale. For practical reasons, Arab numerals are used here. For current earthquakes, the recommended scale is the 1998 EMS (European Macro-seismic Scale) which is an update of the MSK scale more suitable to current buildings (in particular aseismic buildings).Brief description of the M.S.K. 1964 intensity scale degrees(M.S.K.: Medvedev, Sponheuer, Karnik: names of the people who devised it)

This is assessed using a macro-seismic map where all the intensities determined in different places are reported, according to the observations available. The epicentral intensity provides the means for expressing the size of a past earthquake. The intensity is generally greatest at the epicentre and decreases as one moves away from the epicentre. The rate of decrease is greater when the focus of the earthquake is not very deep. Analysis of all the intensities observed on an earthquake provides the means for determining the magnitude which provides an estimate of the energy dissipated at the focus in the form of seismic waves. For current earthquakes, statistical correlations may be established between intensity and magnitude according to the depth of the focus. These correlations in return provide the means for estimating the likely magnitude of past earthquakes.

Epicentral intensity assessment indexes:

Code A: Certain epicentral intensity, estimated according to a tight distribution and an exact value of maximum limited intensities,

Code B: Fairly certain epicentral intensity, estimated according to a less tight distribution and an exact value of maximum limited intensities,

Code C: Uncertain epicentral intensity, estimated according to a scattered distribution and an imprecise value of limited intensities.

Code K: Fairly certain epicentral intensity, resulting from a calculation based on an attenuation law (Sponheuer).

SisFrance is dependent on documentation detailing in writing the effects of past earthquakes. To justify the seismicity in metropolitan France and the areas around it, although SisFrance provides the descriptive bibliography of the events in accordance with a state of knowledge which is updated annually, this site also proposes, with agreement from the Direction des Archives de France (French Archives Division) a certain number of digitised documents which are accessible for display and subject to reproduction rights.